Shock-wave lithotripsy stone fragmentation treatment employs high-energy shock waves to fragment and disintegrate calculi and it can be broadly categorized according to the pattern of energy transfer to the calculi. In this connection lithotripsy can be classified as extracorporeal and intracorporeal.
Shock-wave extracorporeal lithotripsy is a process, which transfers energy needed for stone fragmentation in the form of shock waves from an outside source through body tissue to the calculi. Extracorporeal shock-wave lithotripsy (ESWL) has proven effective in achieving stone fragmentation.
However, since the energy wave transmission is indirect, and in order to carry out the treatment successfully it is required precise directional focusing of the energy at the stone through intervening body tissue. Imprecise focusing might be associated with damaging of the intervening tissues and therefore additional treatments might be required to take care of the damage.
Intracorporeal lithotripsy utilizes a probe advanced to and positioned in proximity to the calculus. The energy, required for fragmentation is transferred through the probe to the calculus and the treatment process can be visualized during fragmentation. The mode of energy transfer may be different and accordingly the intracorporeal lithotripsy techniques are divided into following groups: ultrasonic, laser, electro-hydraulic, electro-impulse and mechanic/ballistic impact.
The last group comprises, for example, detonating an explosive near the stone and causing the shock wave generated by the explosion to act directly upon the stone and crush it into pieces. An example of such technique is disclosed in U.S. Pat. No. 4,605,003, referring to a lithotriptor comprising an inner tube inserted within an outer slender tube and provided with an explosive layer or a gas-generating layer. By the blasting of the explosive layer or the gas-generating layer, the outer slender tube or the inner tube is caused to collide with the stone and crush it.
An example of mechanical impact technique can be found in U.S. Pat. No. 5,448,363 in which is disclosed an endoscopic lithotriptor provided with a hammer element to periodically strike the stone. The hammer element is pneumatically driven by a linear jet of air causing it to swing through an arc about a pivot to impact an anvil.
There are known also many other patents, disclosing lithotriptors, which operation is based on mechanic/ballistic principle, e.g. U.S. Pat. No. 5,722,980, U.S. Pat. No. 6,261,298.
An example of laser technique is described in U.S. Pat. No. 4,308,905, concerning multi-purpose lithotriptor, equipped with laser light-conducting fibers, through which the energy required for crushing the stone is conducted.
It should be pointed out that applying energy by laser is used not only for lithotripsy destroying of stones but also for destroying of other formations, e.g. abnormal tissues causing arrhythmia. An example of this procedure is disclosed in U.S. Pat. No. 6,264,653.
Ultrasonic technique is relatively popular and because of its safety and usefulness is widely accepted. According to this principle ultrasound probe emits high-frequency ultrasonic energy that has a disruption effect upon direct exposure to the stone.
Direct contact of the probe tip and stone is essential for effectiveness of ultrasonic lithotripsy. This technique is implemented in many lithotriptors, e.g. as described in U.S. Pat. No. 6,149,656.
Electro-hydraulic technique utilizes electric discharge, ignited between two electrodes disposed within the probe and producing shock wave, expanding towards the calculus through liquid phase, which surrounds the calculus. In the literature electro-hydraulic lithotripsy is defined as the oldest form of “power” lithotripsy. The electro-hydraulic lithotriptor releases high-energy impulse discharges from an electrode at the tip of a flexible probe, which is placed next to the stone. Since the discharge takes place within liquid phase the calculus is destroyed by virtue of combination of energy of the shock wave, caused by the discharge, hydraulic pressure of the surrounding liquid and collision of fragments in the liquid flow.
Below are listed some references, referring to intracorporeal lithotripting devices, utilizing the electro-hydraulic principle.
A typical electro-hydraulic lithotriptor is described in CA 2104414. This apparatus is intended for fracturing deposits such as urinary and biliary calculi as well as atherosclerotic plaque in the body. The lithotriptor comprises a flexible elongated guide member adapted for insertion within the body, means for supplying a working fluid, a hollow tube mounted on the distal end of the probe, means for initiating an electric spark within the hollow tube from an external energy source, capable of generating pulsed shock waves in the working fluid for impinging the stone and a nozzle, which is made of shock and heat resistant material and mounted on the distal end of the guide member. The nozzle is capable of directing the shock waves to a focal point for impinging the stone. The lithotriptor is provided also with optical viewing system.
In U.S. Pat. No. 2,559,227 is disclosed an apparatus for generating shock. The apparatus comprises a truncated ellipsoidal reflector for reflecting the shock waves and a cavity constituting a chamber for reflecting said shock waves. The cavity has the same truncated ellipsoidal shape, while one of the two focal points of the ellipsoid is disposed in the cavity opposite the truncated part. The cavity is filled with a liquid for transmitting the shock waves, for example oil. The apparatus is provided with a shock wave generator device, conventionally comprising two electrodes disposed at least partly inside said cavity. The two electrodes are arranged to generate an electric arc discharge at the focal point located in the cavity opposite the truncated part. The apparatus has also means for selectively and instantaneously delivering an electric voltage to two electrodes provoking electric arc discharge between said electrodes thus generating shock waves propagating through the liquid contained in the cavity.
The electrodes are made of highly conductive material such as copper or brass and are mounted on an insulator with possibility for adjusting the spacing therebetween.
In DE 19609019 is described an impact probe, provided with at least one electrode guided in the tube. The electrode acts on the object when the probe is longitudinally moved in the direction of the object e.g. a stone. Electro-hydraulic pressure wave is produced at the free end of the probe.
In U.S. Pat. No. 5,254,121 there is disclosed method and device for removing concretions within human ducts as the urethra or kidney. The device includes a flexible probe insertable through the human duct so that a tip thereof is juxtaposed against the concretion. The probe includes a positive electrode extending coaxially within the conduit and embedded in a solid insulation material. A negative electrode is coextensive with and outwardly encircles the positive electrode.
Relatively recently there have been developed medical lithotriptors which operation is based on so-called electro-impulse principle. This principle was adopted from mining technology, where it has been used for so-called high-power electro-impulse destruction of materials. This principle is based on the phenomena that applying of electrical impulses with the rise time of not more than 500 nanoseconds to two electrodes positioned on a solid mineral material immersed in water is associated with producing discharge, which does not propagate through the surrounding liquid medium, but rather through the bulk of the solid body itself. The electro-impulse technology was developed in late fifties in Russia and since then it was successfully implemented in such fields like crushing and disintegration of hard rocks and ores in mining industry, destructing of concrete blocks in building industry, drilling of frozen ground and extremely hard rocks, crushing of various inorganic materials, etc.
A survey of this technology can be found in a monograph “Basics of electro-impulse destroying of materials”, by Semkin et al., Saint-Petersburg, Nauka, 1993.
According to this technology two or more electrodes are placed immediate on the surface of a solid body (rock) and very short impulses of voltage U(t) are sent through them. Once an electrical breakdown between the electrodes is initiated, it occurs in the bulk of the solid body and is associated with producing of the breakdown discharge channel that extends within the bulk of the body.
The body itself serves as a medium to promote propagation of the electrical breakdown rather than the surrounding medium. Extension of the discharge channel through the body is accompanied by mechanical stresses, which stretch the body and destroy it as soon as the tensile strength of the body is exceeded.
In fact in the process of electro-impulse destroying the initiation and propagation of the discharge is similar to a micro explosion within the body.
It can be readily appreciated that since tensile strength of a rock is at least an order of magnitude less than its compressive strength, the electro-impulse crushing is associated with consumption of much less energy, than conventional electro-hydraulic crushing.
It has been also empirically established, that the probability of propagation of the breakdown channel through the body is higher when a very short voltage impulses are applied to electrodes, positioned on a solid body immersed in a liquid medium, since the voltage required for the breakdown within the bulk of the body is less, than the voltage required for breakdown within the liquid medium outside of the body.
Despite the fact that this technology exists for more than 40 years it has been employed mainly in mining and building industry for destruction of very large objects like rocks or concrete blocks as e.g. disclosed in WO 9710058.
The electro-impulse technology was only recently employed in medicine for lithotripsy treatment of calculi and a lithotriptor implementing this technology has been devised. This lithotriptor is manufactured by the company Lithotech Medical Ltd., Israel and is commercially available under the name Urolit. The method and apparatus for electro-impulse lithotripsy is disclosed in International application PCT/IL03/00191.
It should be pointed out that although the present invention is primarily an improvement referring to electro-impulse lithotripsy, nevertheless it can be implemented in other lithotripsy methods based on the principles listed above.
One of the problems associated with functioning of a lithotriptor energized by pulsed energy is erosion and mechanical wear of its probe. When pulses of energy are supplied to the working head of the probe its forwardmost end wears and damage can be caused to insulation of electrodes. As soon as the damage reaches certain limit repetitive use of the probe becomes ineffective, in the worst case, or it can be even dangerous for the personnel and for the patient. Therefore service life of the lithotriptor probe should not be too long and there exists certain limit, beyond which the probe has to be replaced.
There exist some prior art solutions attempting to cope with the problem of safety due to limited service life of the lithotriptor probe. The known in the art solutions are based simply on prolongation of the probe service life by using strong, wear resistant material for insulating the electrodes.
So, for example, in DE 3927260 there is disclosed electro-hydraulic probe, which working end is manufactured from ceramics having high mechanical strength.
In U.S. Pat. No. 5,254,121 is disclosed electro-hydraulic probe employing hard ceramic insulation around the electrodes, which reduces rate of wear and the working head of the probe is designed to reduce the influence of the discharging energy on electrodes.
In JP 3295549 is described electro-hydraulic lithotriptor with electrodes insulated by ceramic coating.
An alternative approach is based on controlling supply of energy supply to the probe to prevent achieving certain preset limit; otherwise operating of the system automatically terminates.
In EP 467137 is disclosed laser lithotriptor in which the energy emitted during operation of the laser is measured and controlled so as to keep it within a certain range.
The laser lithotripter comprises a calibration unit, a monitor unit and a measuring and control unit. During the dedicated calibration step, the energy emitted at the distal end of the probe is measured by the calibration unit and is compared with the energy emerging from the laser and measured by the monitor unit. During the treatment step, the measurement unit and control unit controls the laser operation on the basis of the energy values determined and set during the calibration step and on the basis of the current values determined by the monitor unit. In this system there is possible to control the preset operating parameters of laser during the instant treatment session such that the energy emanating from the probe does not exceed certain value which has been set at the calibration step. This principle of operation however would not be suitable for lithotriptors employing wearable probes since it does not take into consideration the energy supplied during the previous treatment sessions. The energy supplied during previous treatment sessions could cause wear to the probe before the instant session and therefore it should be taken into consideration for accurate estimation of the remaining service life of the probe.
In U.S. Pat. No. 6,264,653 is disclosed ablation catheter for creating long continuous lesions at targeted anatomical sites. The catheter is provided with a plurality of electrodes heated by pulsed radio frequency energy which is supplied to electrodes sequentially or continuously. The system and method is described which enables gauging the amount or quality of the contact between body tissue and one or more electrodes by counting the number of pulses delivered to a particular electrode and comparing it to the number of pulses supplied to at least one other electrode.
In wearable probes and especially those employed in electro-hydraulic or electro-impulse lithotriptors where pulsed energy is supplied to the probe it would be desirable to monitor the probe's service life continuously and assess it depending on the amount of previously delivered to the probe energy. This would allow deciding whether the remaining service life of the probe is still sufficient for its efficient and safe operation during the further treatment session or not. Furthermore, such monitoring would allow alerting and timely termination of the lithotriptor operation as soon as remaining service life of the probe approaches certain preset limit.